Hygrometer elements



ILOGAIRITHM OF RESISTANCE (OHMS) Oct, 9, 1962 F. E. J O N E S HYGROMETERELEMENTS Filed July 23, 1959 7O RELATIVE HUMIDITY,

INVENTOR FRANK E JONES ATTORNEY 3,058,979 HYGROMETER ELEMENTS Frank E.Jones, Washington, D.C., assignor to the United States of America asrepresented by the Secretary of the Navy Filed July 23, 1959, Ser. No.829,162 1 Claim. (Cl. 33835) (Granted under Title 35, US. Code (1952),see. 266) The invention described herein may be manufactured and used byor for the Government of the United States of America for governmentalpurposes without the payment of any royalties thereon or therefor.

The present invention relates to electric hygrometer elements and moreparticularly to electric hygrometer elements incorporating particularhumidity sensitive materials.

Since the electrical resistance of humidity-sensitive materials isaltered in some relationship to the change in relative humidity of theatmosphere about it, it is wellknown in the art that by measuring suchchanges in resistance humidity determinations can be made.

This invention has been the outgrowth of investigations directed towardproducing an electric hygrometer for use in radiosondes and similardevices. To be adaptable to such use the response time must be verysmall in order to record the rapid changes in humidity during ascents atspeeds in excess of 1000 feet per minute. Further, for radiosonde use,the response times at low temperatures are critical. As the temperaturedecreases the lags in response increase and in the case ofhumidity-sensitive materials in which the change in electricalresistance is dependent on the transfer of water into or through thematerial (volume conductivity) such time lags may well be so great as torender the readings of little value.

Among the prior art methods of measuring humidity by means of humiditysensing devices employing changes in electrical resistance are thefollowing:

1) Aqueous salt solutions such as lithium chloride, calcium chloride andzinc chloride, or solutions of acids such as sulfuric or phosphoricacids are mixed with one or more binders such as polyvinyl acetate,polyvinyl alcohol, gelatin, pectin and agar-agar and such mixturesubsequently deposited by dipping or spraying on an insulator havingmetallic electrodes mounted thereon. The concentration of the saltsolution, and hence the conductivity, changes with relative humidity.

(2) A plastic binder, such as hydroxyethyl cellulose, is mixed with thefollowing: a humectant-typ'e plasticizer such as polyethylene sorbitol;a non-ionic dispersing agent such as alkyl aryl polyether alcohol, and aconductor such as powdered carbon. The resulting mixture is deposited bydipping, spraying, or painting on an insulator having electrodes mountedthereon. The plastic binder expands and contracts with changes inrelative humidity changing the distances between the conductingparticles and hence changing the overall resistance.

(3) Natural or synthetic fibers (such as cotton, silk, nylon or rayon)or fabrics made from natural or synthetic fibers (such as cloth, paperor asbestos) or natural organic materials (such as human or horse hair)are impregnated with saturated aqueous solutions of salts such aslithium chloride or calcium chloride with or without binders. Theresistance of the impregnated material varies with relative humidity.

(4) Wicks of fabric such as glass wool are impregnated with a saturatedsolution of salt such as lithium chloride and a bifilar wire winding iswound around such an impregnated wick. The wick is heated automaticallyby passing an electric current between the parallel wires until thetemperature of the wick and the temperature of the saturated saltsolution is such that there is no gain or atent dice 3,058,079 PatentedOct. 9, 1962 2 loss of moisture from or to the ambient atmosphere. Thetemperature of the wick, measured by a resistance (or by a liquid-filledthermometer in the center of the wick) is a measure of the vaporpressure of the ambient atmosphere and is, therefore, a measure of therelative humidity.

(5) Porous materials such as paper, leather, regenerated cellulose,underfired clay, plaster of paris or brick all of which absorb watervapor have been used. The resistance of such materials varies withrelative humidity.

(6) Non-porous insulating materials such as glass, quartz, porcelain andplastics are also used since the surface resistivity of such materialsvaries with relative humidity.

(7) Evaporated films of potassium metaphosphate, sodium carbonate,calcium sulfate, cuprous iodide, potassium sulfate or sodium chloridedeposited on an insulating base are used to measure relative humidit Asto the disadvantages of the prior art methods, methods 1) through (5)have poor response times. The reason for this slow response to changesin relative humidity is that the water vapor has to be absorbed into ordiffused through a volume of material (volume conductivity). Method (6)is not reproducible and involved instrumentation has been required sincethe resistances to be measured are very high. Method (7) has thedisadvantages that very high resistances must be measured requiringspecial instrumentation; that the lower limit of relative humidity thatcan he measured is set by the surface resistance of the substrate, andfurther that prolonged exposure to high relative humidities results inundesirable shifts in the resistance-relative humidity relationship.

Thus, an object of the invention is to provide a hygro-meterincorporating particular humidity-sensitive materials thereby having -aresistance range which can be conveniently handled without the use ofspecial circuitry and without limitations due to the parallel resistanceof the substrate.

A further object of the invention is to provide a hygrometerincorporating particular humiditysensitive materials thereby enablingthe attainment of low resistance but very thin films of such materials.

Another object of the invention is to provide a hygrometer of highsensitivity and precision.

Still another object of the invention is to provide a hygrometer capableof rapid response times even at low temperatures (below 40 C.).

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same become better understood byreference to the following detailed description when considered inconnection with the accompanying drawings wherein:

FIG. 1 shows a plan view of a preferred embodiment of the invention witha portion thereof broken away;

FIG. 2 shows a section taken on line 22 of FIG. 1;

FIG. 3 shows a plan view of a modification of the device with a portionthereof broken away;

FIG. 4 shows a section taken on line 44 of FIG. 3, and

FIG. 5 shows a curve indicating relative humidity plotted as a functionof the logarithm of the resistance variation of the hygrometer element.

Referring now to the drawings and FIGS. 1 and 2 in particular, thehumidity sensitive element 11 comprises a base or substrate member 12which may be made of glass, quartz, polystyrene or similar plastics orother electrically insulating materials. It would be possible as well touse an electrically conducting material such as a metal providing thatit be covered by a thin electrically insulating layer. The substrate iscleaned chemical-1y using water, detergents and organic solvents, thelatter being applied by dipping or by the use of a vapor degreaser.

The conducting or electrode materials 13, 13 which are adhered to thesubstrate 12 can be any of a number of conducting materials such asmetals, alloys of metals, solid solutions or combinations of metals. Theelectrodes 13, 13 can be in the form of parallel strips or can be formedin configurations specially designed to adjust the resistance values ofthe device. Palladium, gold, silver, and chromium have, for example,been deposited by evaporation. In the preferred embodiment shown, thecloselyspaced intermeshing comb electrode configuration was formed by afine-line etching process. An evaporated film of chromium (or othersuitable material) is deposited on the substrate. This film is coveredby a photosensitive resist. This resist is exposed to ultraviolet lightthrough a photographic negative of the electric pattern. The resist isthen developed and the unwanted resist washed off leaving a fine line ofelectrode film exposed. This fine line is removed by electroetching thefilm in an electrolytic solution.

One pattern successfully employed is of the general configuration shownin FIG. 1 in which each of the two combs 13, 13 have 122 teeth 14, eachtooth being 0.008 inch wide and inch long. The separation 16 between theintermeshed teeth is 0.006 inch and the overall length of the pattern is3 inch.

By employing such a closely-spaced electrode configuration and in effectproviding very long electrodes separated by a very small distance thenet result is to markedly reduce the electrical resistance of thehygrometer.

The humidity-sensitive material 17 is deposited on the substrate byvacuum evaporation or sublimation although deposition can be made bychemical or other means. Actually the prime requisite is the recognitionand employment of a series of criteria in the selection of thehumidity-sensitive material.

In isolating these criteria the theoretical approach has been toconsider the layer of sensitive material a solid variable resistor andto consider the dependence of the resistance of this solid on relativehumidity as being the result of interaction between the solid and watervapor.

The water vapor molecule is a dipolar molecule (i.e., the centers ofgravity of the positive and negative charge do not coincide) having alarge permanent dipole moment (the product of the magnitude of eithercharge by the distance between the centers of gravity of the charges).Substances can be considered to consist of atoms, molecules, and ions.An atom, molecule, or ion may be polarized by induction in an externalelectric field. The induced dipole moment is proportional to theelectric field intensity. The proportionality constant, or, is calledthe polarizability of the particle. The water vapor-resistor system maytherefore be considered to consist of polar and polarizable molecules ofwater vapor and polarizable atoms, molecules, or ions of the resistormaterial.

It can be shown that the water vapor molecules and the particles of theresistor will be mutually attracted by intermolecular forces. The forcesinvolved are the van der Waals forces. The van der Waals forces areattributed to three effects: the orientation effect, the inductioneffect, and the dispersion effect.

The van der Waals forces are the forces which are effective in physicaladsorption. Physical adsorption is the weak interaction between solidsand gases in which the gas condenses on the surface of the solid. Inaddition to physical adsorption there is the possibility of chemicaladsorption, a strong interaction similar to chemical reaction. Chemicaladsorption ischaracterized by irreversibility in most cases and slowresponse. These two characteristics make chemical adsorption unsuitablefor an electric hygrometer; therefore, physical adsorption systems onlywill be considered.

In physical adsorption of gases on solids, molecules in the gas or vaporbecome attached to, adsorbed on, the surface of the solid. Physicaladsorption takes place at very great speed and is reversible (i.e.,water vapor adsorbed at high relative humi-dities is removed, desorbed,at low relative humidities).

The generalization can now be made that the ability of a substance toadsorb Water vapor by physical adsorption depends upon the magnitudes ofthe permanent multipole moments and the polarizabilities of theparticles of the substance. This generalization is applicable tononionic substances (metals, for example) as well as ionic substances,and to liquids and gases as well as the solid state. Therefore, animportant result of the investigation of the resistor-water vaporinteraction is the establish ment of the polar and polarizable nature ofthe adsorbing substances as important criteria to consider in theselection of materials for use in humidity sensors.

Many materials experience changes in surface electrical conductivitywhen exposed to humid air, i.e., when water vapor is adsorbed ordesorbed. This property can be exploited in producing an hygrometer inwhich the very desirable characteristics, extremely rapid response andreversibility, of physical adsorption are inherent. In practical casesof physical adsorption in which response is not found to be practicallyinstantaneous, the slow step in the process is diffusion. Since rapidresponse is essential in radiosonde hygrometers, the use of a very thinfilm of the adsorbing material providing very short diffusion pathswould minimize the effect of diffusion, if it were present.

From the preceding, some criteria can be set up for the selection ofmaterials to be used in the form of thin films as the sensitivematerials for electric hygrometers. These criteria are presented below:

Electronic Polarizabilities (Units 10 cm?) Barium fluoride 3.976 Cesiumfluoride 3.664 Cadmium iodide 14.61 Cuprouschloride 4.628

Thallium iodide 12.238 Strontium sulfate 6.500

Equivalent Conductances of Separate Constituent Ions (Units Ohm- 0 C.)

Vapor Pressure of the Material All of the materials covered specificallyherein have high enough vapor pressures that evaporation can proceed atconveniently low source temperatures i-.e.

Cesium fluoride .1 millimeter of mercury at 7 12 C. Cadmium iodide 1millimeter of mercury at 416 C. Cuprous chloride -1 millimeter ofmercury at 546 C.

Ease of Evaporation All of the materials have been convenientlyevaporated from molybdenum boats without requiring excessive power andwithout excessive reaction with the boat material. Films of suitablethicknesses have been deposited in conveniently short times.

Adhesion of the Material to the Substrate The materials covered here-inadhered well enough to the substrate to allow the films to be usedwithout special handling procedures. Barium fluoride, for example,adhered well enough to the glass substrate to allow immersion in waterwithout removal of the film.

Temperature Coefiicient of Expansion The temperature coeificient ofexpansion of the film should not be incompatible with that of thesubstrate. The smaller the differential between the coefficient ofexpansion of the substrate, the smaller the strain to which the bondbetween film and substrate will be subjected as the result offluctuations in temperature. The coeificient of the film is dependent onthe conditions under which the evaporation is made and is thereforesubject to some adjustment.

The conditions existing in the vacuum chamber before,

-during, and after the evaporation (deposition) determine the initialstructure and physical properties of evaporated thin films. Recognitionof the influence of these conditions and the setting up of controls overthem will enable one to produce films reproducibly and with somepredetermination of structure and properties. The following is a list ofsome of the influencing parameters and conditions which can becontrolled or at least measured throughout the film production processand subsequent treatment:

Structure of the Deposited Film The structure of evaporated filmsdepends upon the conditions under which the evaporation is made.However, some materials form films which are inherently unsuitable tothis application due to porous structure, granulation, or some otherunfavorable characteristics. The barium fluoride films and others of thematerials covered specifically herein have satisfactory structure asindicated by rapid response and low hysteresis.

Porosity of the Films and Solubility These two properties are used toindicate the unsuitability of a material. That is, if the film is porousit will be unsuitable due to excessive hysteresis, irreversibility, andexcessive response times; if the film is easily soluble in water, theresistance of the hygrometer will change permanently upon exposure tohumid air and the film might wash off the substrate. Theseproperties'are, therefore, not positive, but negative criteria.

Solubilities of materials in water are tabulated in handbooks, etc., andcan serve as a guide in eliminating materials. However, since evaporatedfilms have properties which are not the same as those of the bulkmaterials, the solubilities of the films can be different than those ofthe bulk materials.

Solubilities of some of the materials covered by this application arelisted below:

(Units grams per 100 milliliters) Electrical Resistance-RelativeHumidity Characteristics of the Resulting Device It is preferable foruse in radiosondes that the hygrometer have a resistance rangecompatible with the 6 radiosonde circuitry and also high sensitivity. Byselec tion of materials based on the criteria discussed above,hygrometers with suitable resistances and sensitiviy can be produced.The resistance can be further lowered by using a suitable electrodeconfiguration.

Having applied the above criteria the following materials have beenisolated, tested and found successful: barium fluoride cerous fluoride,didymium fluoride, lanthanum fluoride, neodymium fluoride, cesiumiodide, cadmium iodide, cuprous chloride, thallium iodide, lead iodide,strontium sulfate and lead sulfide.

As an indication of the greatly improved performance of materialsselected by the above developed criteria the following are responsetimes reported for barium fluoride hygrometers: at approximately 25 C.the response times are of the order of 0.1 second; at 20 C., about 1.5seconds; and at -40 C., about 3 seconds. Since for a radiosonde use, theresponse times at low temperatures are critical this data Will indicatethe suitability of the present hygrometers for such an application.

The following serves to illustrate the production of a typicalhumidity-sensing device as is shown in FIGS. 1 and 2. A glass plate 12such as a standard microscope cover slide glass is cleaned by standardprocedures and placed in the vacuum chamber of a vacuum evaporator (notshown). A glow discharge is maintained at a pressure of approximately 20microns of mercury for about 20 minutes in the vacuum chamber to furtherclean plate 12. A thin film of metal such as palladium or chromium isdeposited on the plate by vacuum evaporation or sublimation and aclosely-spaced electrode comb configuration 13, 13 is produced by thefine-line etching process described above. A humidity-sensitive material(selected from the group disclosed above) such as barium fluoride isheated in a vacuum chamber and a thin film 17 of the material isdeposited over the exposed glass plate 12 and the electrodeconfiguration 13, 13 by vacuum evaporation or sublimation. Theelectrical resistance of the device varies with relative humidity andthis resistance can be measured by contacting electrodes 13, 13 atcontacts 18, 18 with leads 19, 19 connected to a resistance measuringdevice (not shown).

In the modification 26 shown in FIGS. 3 and 4 electrodes 27, 27 areplaced over the humidity sensitive layer 28 which in turn covers thesurface of substrate 29. Since the performance of the selectedhumidity-sensitive material depends upon surface attachment oradsorption of the molecules of water vapor, the results obtained in thecase in which the electrodes are placed over the humiditysensitivematerial are in substantial accord with the re sults obtained in thecase in which the humidity-sensitive material is deposited over theelectrodes. However, due to the care which must be taken to avoid damageto layer 28 during the placement of electrodes 27, 27 the constructiondisclosed in FIGS. 1 and 2 are the preferred construction.

The portion of the humidity-sensitive layer 28, exposed in gaps orseparations 31 between electrode teeth 32 serves to register changes inmoisture content of the atmosphere. Leads 33, 33 attached to electrodecontacts 34, 34 connect to a resistance measuring device (not shown).

If a device such as is disclosed in FIGS. 1 and 2 employing a bariumfluoride film is tested and a calibration curve is prepared therefor inwhich the logarithm of electrical resistance is plotted against therelative humidity, the resultant curve will be that shown in FIG. 5. Itcan be seen that the electrical resistance of the device varies throughmore than four degrees of magnitude throughout the range of relativehumidity. In this manner the magnitude of variation in resistance whichgives the device high sensitivity can be seen. Such high sensitivityenables measurement of this resistance (and thereby the correspondinghumidity) without the need for special circuitry. In addition theprecision of meausre- 7 ment is increased. It has further been foundthat the hysteresis effects of such a device are very small, adding tothe attainment of satisfactorily repeatable relative humidityindications.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claim the invention maybe practiced otherwise than as specifically described.

What is claimed is:

In an electric hygrometer element having an electrically non-conductivesurface, a pair of electrodes mounted on said surface, ahumidity-sensitive material arranged as an adherent film in the gapbetween said electrodes and in electrical contact with said electrodes,said material having an electronic polarizability in the range betweenabout 3 10- cm. to 18.0 l0 cm. and having 8 conductanc'es of constituentions in the range between 18 ohm and about 52 ohmthe improvement in saidcombination of a very thin film of barium fluoride arranged as theadherent film in the gap, said film: being non-granular, non-porous anddiflicultly soluble in water,

whereby on exposure of said film to moist atmosphere a rapidlyresponsive indication of humidity is obtained.

References Cited in the file of this patent UNITED STATES PATENTS Amduret al. Mar. 3, 1959

